In the scene where Dr. Ryan Stone's (Sandra Bullock) leg is entangled in Soyuz's parachute cords and she is tethered with Matt Kowalski (George Clooney) through a strap, there is tension in the strap, suggesting act of some external force. Where is this force coming form that is causing tension in the strap and pushing both Dr. Ryan and Matt outwards?

I'm guessing that the tension might be coming from centrifugal force, but immediate events before this scene, as well as other parts of ISS in same scene do not suggest presence of such force (or presence of any kind of rotation).

7 Answers
7

While I for myself am not sure if there was any tension (especially a continuous tension pulling him away even after his motion has been stopped by Dr. Stone), this interview with a real astronaut suggests there wasn't, especially this section:

We see quite a few worst-case scenarios in the film. George Clooney ends up untethering himself from his crewmate and literally floating
away into space. Is dealing with that scenario part of your training?
Or does that not even come up?

Actually, I really couldn't understand the tension there. Sandra has
her leg caught in some parachute rigging, and she's holding on to
George by his hand. I think all he would have needed to have done
would be to crawl up on her, and basically rope-climb up toward the
space station. Or just lightly pull and he could have flown himself up
to the space station. There would be no continuing motive force to
continue to pull him away. So that physically didn't work for me.

So when George he let go of her hand, if it was in real space, he wouldn't have been pulled away.

He would have just floated there.

So according to the above interview the answer to your question would be "nothing".

And again the default answer to all those Gravity questions: "it wasn't as scientifically accurate as it might have suggested". (just don't let that ruin this amazing movie for you, it wasn't a documentary anyway).
– Napoleon Wilson♦Nov 12 '13 at 9:14

Yet nobody knows if the astronaut from this interview just didn't understand that scene entirely, but we just have to assume he knows what he's talking about.
– Napoleon Wilson♦Nov 12 '13 at 9:24

Actually, I really couldn't understand the tension there. Sandra has her leg caught in some parachute rigging, and she's holding on to George by his hand. I think all he would have needed to have done would be to crawl up on her, and basically rope-climb up toward the space station. Or just lightly pull and he could have flown himself up to the space station. There would be no continuing motive force to continue to pull him away. So that physically didn't work for me.

I am not the down-voter, but the reason appear to me is that , your answer looks like a link only and doesn't answering it. Best suited to be a comment rather then answer. No offense intended.
– Ankit SharmaDec 11 '13 at 15:39

1

'the film is hugely flawed, scientifically speaking.'.. that's the answer given, paraphrased from the link. I'll put a full quote in to remedy it, but I did include an answer and a reference link. Not sure how much more I can do.
– John Smith OptionalDec 11 '13 at 15:42

During that scene, Stone's leg gets wrapped up in some parachute cabling. She then starts slowly rotating around whatever the cabling is stuck on. Then she grabs the strap attached to Kowalski. She's still rotating. In order to hold onto him, she's providing a centripetal force. After releasing him, she finally rotates into another part of the station, and works her way to the airlock.

I look forward to being able to provide some evidence of this once the film is released on blu-ray.

Basic physics:

We have 4 things of import in this image.

Stone's ongoing motion, which are the lines behind her in my crude drawing.

A, the force that Stone is exerting on Kowalski.

B, the force Kowalski is exerting on Stone. This is equal and opposite to A.Newton's 3rd Law If it wasn't equal and opposite, then Kowalski would be moving toward or away from Stone at this moment.

C, the centripedal force exerted on Stone by the parachute cabling wrapped around her foot. This force is going to be equal to A + the amount of force needed to keep Stone alone moving in the arc.

C is larger here due to it needing to pull the mass of both Kowalski and Stone. Once Kowalski is released, then C just needs to pull Stone, which drastically reduces C's magnitude.

In the scene, while C is large, Stone's leg is slipping out of the parachute cabling. Once C is reduced, this stops, and she soon swings into the hull of the ISS.

Makes sense, though her holding him should deccelerate (is that a word?) his movement until he stops being dragged away, shouldn't it (but I'm far from an expert in orbital mechanics either)?
– Napoleon Wilson♦Nov 12 '13 at 18:01

1

They're both rotating still. Until something stops that movement, she needs to maintain that centripetal force.
– user209Nov 12 '13 at 18:05

I still don't understand why Clooney had to let go. It seemed he could just crawl back or pull himself in
– HuangismDec 11 '13 at 19:19

@Huangism There, I added a drawing and explanation. Further questions can be directed to me or Physics.SE.
– user209Dec 11 '13 at 19:38

2

Having just rewatched it, while your answer makes sense, they (and the stuff they're attached to) don't look as if rotating at all (e.g. when comparing to the earth in the background), but rather standing completely still with Kowalski just being pulled away. But I may still be wrong and am looking forward to you rechecking the scene, too.
– Napoleon Wilson♦Feb 23 '14 at 1:15

My guess is that even though the parachute lines had stopped slipping on Stone's leg, the parachute itself was still slipping through the wreckage, and Stone and Kowalski still had momentum directed away from the wreckage. The friction of the parachute against the wreckage was slowing their momentum and causing the tension. Kowalski was facing the parachute. Maybe he estimated that the friction was enough to gently stop one person but not two, before the parachute pulled all the way through the wreckage, or before the line connected to the escape vehicle pulled taut, at which point the lines might be dislodged from Stone's leg. And rather than trying to explain all that, Kowalski would probably try to be more succinct, which he was.

The film was not really successful in getting this idea across, but I'm sure we can agree it is a difficult scenario to depict. I personally am able to suspend my disbelief that far.

It doesn't help that the director is trying to tell the story using long, uninterrupted takes. He doesn't have the option of the usual tension-building technique of cutting to a closeup of the parachute slipping free of one outcropping, then another, etc.

+1 This is the right answer. Stone was slowed down by the friction of the slipping line in the wreckage. Kowalski obviously had more velocity, which caused an acceleration on Stone. As long as these two (opposite) accelerations exist (that is: as long as the line was slipping), the strap between them is taut.
– NGLNMay 24 '14 at 15:12

I'm not a physicist but basically it comes down to Newton's three laws of motion. Basically, an object in motion will stay in motion unless another force stops it. In space, the only thing that could really stop his momentum would be an object of greater mass, but the two astronauts are approximately the same mass so in effect, he was transferring his momentum to her and causing them both to be in trouble, so he let go.

yeah, only she was caught by the rope which was still attached to the iss. as i said, their momentum in relation to the iss, at least as far as i could tell, had already been halted
– DForck42Nov 28 '15 at 3:28

The problem is you say "as far as I could tell" yet the film and science don't support an assumption that you've made.
– sanpacoNov 28 '15 at 9:45

This is a resourceful answer, but once the tether is taut between them, the damage is done, so to speak. Her inertia is set; him letting go is not going to send her back. This is just how is appears to me. In order to make the plot work like they wanted, they had to pull a few strings (LOL) in terms of artistic license.
– BaodadOct 7 '16 at 21:47

What would have made sense for this setup is if they had a centrifugal force on them (e.g. by making the whole station rotate slowly, maybe caused by a previous hit)

But in the scene it was pretty clear that the station had no visible angular velocity and the slow cascading untanglement of the parachute-cord could not create that continous force, it would need to be a continous slipping untanglement.